1. Field of the Invention
The present invention relates to high-intensity permanent magnets. More specifically, it relates to magnets having a high-intensity working magnetic field that varies in the direction of its orientation.
2. Description of the Prior Art
There are a number of technological fields in which magnetic systems capable of producing large magnetic fields with axial gradients are desirable. For example, electron-beam tubes often require gradient fields for beam focusing and guidance. Such gradient fields typically vary along the beam axis. As another example, an axial field variation of a longitudinal field is required in many micro/millimeter wave sources. In applications where transverse fields are required, as in storage rings and particle accelerators, field tapering in the direction of the beam axis is sometimes necessary to compensate for changes in the axial beam velocity. Still further, in the fields of chemical analysis and spectroscopy, magnetic fields with a linear taper in the field direction are often used to produce a spectral distribution of absorbed or emitted electromagnetic energy.
Those concerned with the development of such systems have long recognized the need for magnetic structures capable of producing gradient magnetic fields of high intensity in a compact structure, i.e. a magnet having a minimum of structural mass and bulk. The present invention fulfills this need.
High-remanence, high-coercivity, permanent-magnet materials, such as those of the rare-earth type, have improved so that it is now practical to make flux sources of extraordinary strength and compaction. Examples of high-intensity, compact permanent magnets may be found in the following references:
Leupold, U.S. Pat. No. 4,837,542, entitled "Hollow Substantially Hemispherical Permanent Magnet High-Field Flux Source for Producing a Uniform High Field;"
Leupold, U.S. Pat. No. 4,839,059, entitled "Clad Magic Ring Wigglers;"
Leupold et al., "Novel High-Field Permanent-Magnet Flux Sources," IEEE Transactions on Magnetics, vol. MAG-23, No. 5, pp. 3628-3629, Sep. 1987;
Leupold et al., "A Catalogue of Novel Permanent-Magnet Field Sources," Paper No. W3.2, 9th International Workshop on Rare-Earth Magnets and Their Applications, pp 109-123, Aug. 1987, Bad Soden, FRG;
Leupold et al., "Design applications of magnetic mirrors," Journal of Applied Physics, 63(8), 15 Apr. 1988, pp. 3987-3988;
Leupold et al., "Applications of yokeless flux confinement," Journal of Applied Physics, 64(10), Nov. 15, 1988, pp. 5994-5996; and
Abele et al., "A general method for flux confinement in permanent-magnet structures," Journal of Applied Physics, 64(10), Nov. 15, 1988, pp. 5988-5990.
Additionally, magnets of the type described herein may be found in my following copending U.S. Patent Applications that are incorporated herein by reference:
U.S. Ser. No. 654,476, filed Feb. 13, 1991, entitled "High-Power Electrical Machinery;"
U.S. Ser. No. 650,845, filed Feb. 5, 1991, entitled "High-Power Electrical Machinery with Toroidal Permanent Magnets;"
U.S. Ser. No. 709,548, filed Jun. 3, 1991, entitled "High-Field Permanent Magnet Flux Source;"
U.S. Ser. No. 892,104, filed Jun. 2, 1992, entitled "Magnetic Field Sources Having Non-Distorting Access Ports," and
U.S. Ser No. 892,093, filed Jun. 2, 1992, entitled "Field Augmentation in High-Intensity Magnetic Field Sources'".
These references show a number of high-intensity permanent magnets having a variety of different compact shapes. In general, these magnets have a shell of magnetic material and a cavity in which a uniform working field is located. Access ports of various sizes, shapes and locations pass through the shell and communicate with the uniform field in the cavity.
Salient among these magnets are cylindrical ("magic ring") and spherical ("magic sphere") magnetic shells in which the direction of remanent magnetization in the shell changes as a function of a polar angle while its intensity remains constant. These magnets produce in their cavities uniform, polar-axial transverse fields. Theoretically, there is no limit to the cavity fields attainable in a magnet of this type if one is willing to employ enough magnetic material of sufficiently high coercivity to retain its magnetism in the face of the high distorting fields engendered by the structure.
In practice, it is difficult to produce a spherical or cylindrical shell having a remanent magnetization the direction of which continuously varies. Consequently, such shells are typically constructed from segments that are each uniformly magnetized. When nested, the segments form a magnetic shell. In the case of a segmented cylindrical shell, the angular direction of magnetization usually changes abruptly by 4.pi./N between adjacent segments, where N is the number of nested segments.
A working field produced by a segmented shell suffers surprisingly little from the approximation by segmentation. For example, if a cylindrically shaped shell is divided into sixteen segments, it produces a magnetic field of over 97% of that produced by a continuous structure. Even with a coarse approximation of only eight segments, 90% of the ideal field is realized. Specifically, a segmented spherical shell having an outer radius of 3.3 centimeters (cm) that is made of a magnetic material having a remanence of ten kilogauss (kG) can produce a field of sixteen kilo-oerstead (kOe) in a spherical cavity having a radius of only 1.0 cm. The shell would have a mass of only 1.1 kilograms. Similar performance is obtainable from cylindrical and hemispherical structures.
Although such compact magnets have served to produce high-intensity magnetic fields, it is recognized that such fields are normally of substantial uniformity. As indicated above, an important need also exists for compact magnets that produce variable fields of comparable intensity.